Refrigeration/heatpump systems commonly employ a compressor with an outlet and a suction inlet connection by at least one closed loop wherein a fixed amount of refrigerant is successively circulated, condensed and evaporated. Expansion devices are routinely positioned in a main circulation loop between the condenser and the evaporator in order to create, in conjunction with the suction from the compressor inlet, a pressure dropped condition at the evaporator and the main line leading to the suction inlet.
When the refrigeration system is shut off, this pressure dropped condition causes liquid and vapor-phase refrigerant in the system to migrate from the high pressure areas in the system into the compressor until the pressure is equalized throughout the system. The liquid and the vapor-phase refrigerant collects in the oil within the cylinders of the compressor whereupon, in the absence of compressor heating (typically provided by an internal electric heating element) the vapor condenses to form additional liquid.
Liquid refrigerant has a particular propensity for oil, most of which remains within in the compressor. The architecture of the particular refrigeration system may also facilitate migration of refrigerant into the compression chamber of the compressor.
Many air conditioning systems place the compressor outside of the area to be cooled. When the system is shut down, in the evening for example, liquid-phase refrigerant will migrate from the warmer area inside to the colder area outside. Additionally, migration of fluid into the compressor is caused by the lower vapor pressure in the evaporator compared to the vapor pressure in the compressor cylinder which contains oil. Liquid-phase refrigerant can also surge back into a compressor during defrosting of refrigeration systems, or when fans or filter systems fail.
The amount of refrigerant migration into the compression cylinders of the compressor depends upon a number of factors, such as amount of refrigerant, temperature, amount of oil, and length of time that the system is shut off. When power is restored to the system, the non-compressible liquid-phase refrigerant that has migrated to the compressor cylinder cannot escape through the compressor outlet during a compression cycle, typically resulting in catastrophic failure (slugging) of the compressor, as by bending of the crankshaft, for example.
As the temperature of the operating compressor increases, the liquid refrigerant boils off, carrying the oil in the compressor with it, and if the compressor continues to run, it may do so without sufficient lubrication. As the piston moves through its range within the cylinder, a point is reached at which the piston cannot move any more against the incompressible liquid refrigerant/lubricating oil mixture. At this point "slugging" leading to immediate catastrophic failure of the compressor ensues. As mentioned, the crankshaft may bend or break, or the connecting rod may bend or break. Whatever the failure mode, compressor slugging usually results in replacement of the compressor unit, particularly if it is of the ubiquitous hermetically sealed variety.
Several devices have been employed in the prior art in order to prevent compressor slugging. Solenoid valves have been placed in the output and suction input lines of compressors. The valves are closed to prevent freon refrigerant from entering the compressor when the system is shut off. The most common device used to prevent slugging is a compressor crankcase electrical heating element. Various types of crankcase heaters exist and all require electrical power for operation. The heaters may operate continuously when the compressor is turned off in which case the heater maintains the oil in the compressor at a temperature sufficient to maintain the refrigerant in vapor/gas phase and thereby continuously repel the liquid-phase refrigerant from the compressor. Other crankcase heaters are only used when the compressor system is to be reactivated. Extended time periods of pre-heating may typically be required in order to drive the liquid out of the compressor before the compressor system can be reactivated in a refrigeration cycle, depending upon the ambient environmental temperature conditions.
Another type of crankcase heater operates by maintaining a partial supply of alternating current to the compressor via a suitably sized capacitor. This trickle current flow is dissipated as heat within the non-rotating motor and thereby keeps the compressor warmed. All crankcase heaters require electrical power; therefore all of the heaters fail to protect the compressor when power is suddenly restored following a power failute of a sufficiently extended duration to permit liquid-phase refrigerant to enter the compression chamber of the compressor.
As can be seen from the above summary of the prior art, an unsolved need exists for an inexpensive method that is not dependnet upon an uninterruptable electrical power source for reliable prevention of slugging within a compressor.